The present invention relates to a positive displacement internal combustion engine with multiple stages of compression and expansion reaching very high pressures and having density control of engine torque.
It is known that increasing the expansion ratio of a reciprocating internal combustion engine extracts more energy during the expansion of the combustion gases. Therefore, the thermodynamic efficiency increases as the expansion ratio increases. For constant volume combustion, the theoretical thermal efficiency equals: EQU 1-1/r.sup.(k- 1) (1)
where r is the expansion ratio and k is the adiabatic expansion coefficient, which for air at room temperature is 1.4.
Conventional Otto cycle engines usually have the same compression and expansion ratios, which are selected so that the fuel/air mixture is compressed to a point below which spark ignition does not cause detonation. Detonation depends on the anti-knock characteristics of the fuel and on the combustion chamber design. The ratio is usually about 7 in automobile engines using regular fuel, but the ratio can be over 10 in aircraft engines. Diesel engines also have equal compression and expansion ratios, but the air is compressed to a point where the injection of fuel causes ignition.
Multiple staging has been known as a way of using more of the available energy left after expansion in an earlier stage. Early multi-staging is taught in conjunction with steam engines. Drautz, U.S. Pat. No. 423,224 (1890) is an example of a multi-stage steam engine. The last stage may expand steam to sub-atmospheric pressure. Turbo machinery has also used multi-staging. In reciprocating aircraft engines, the supercharger is driven by elevated pressure exhaust gas at high temperature to drive a compressor to compensate for decreased air density at high altitude Multi-staging has also been proposed for positive displacement rotary engines such as that disclosed in Hubers, U.S. Pat. No. 3,783,615 (1974). Multiple staging was important in rotary engines so that the engines could reach conventional compression ratios because positive displacement mechanisms in rotary engines have low efficiencies except at low pressure ratios.
Thermodynamic advantages of multi-staging are understood, but the problems in achieving these advantages in a practical manner have prevented their implementation. Theoretical problems such as detonation, increased heat transfer losses, large mechanical forces, increased friction losses, transfer losses, large size, complexity and many other smaller problems have contributed to the lack of interest in developing such an engine. Moreover, many of the tradeoffs in basic engine design took place when the fuel was very inexpensive and there was less demand for increased engine efficiency.
Pollution, heat dissipation and low efficiency for power variation are among the potential problems confronting present engineers. Engine efficiency also suffers from heat dissipation, which can increase as compression and expansion ratios increase. The heat loss represents energy not available for useful work. Conventional engines have no means to minimize the loss of available work with heat loss. Heat loss not only results in theoretical loss of available energy, but the engine must also drive equipment to cool heated engine parts.
Throttling the charge (the amount) of fuel/air mixture flowing to the combustion chamber at any time is the conventional way of varying engine torque and power. Throttling results in large efficiency losses, which are caused by fluid flow losses and pumping. To compensate for these losses, vehicles use transmissions of up to five speeds for automobiles and up to twenty speeds for large trucks.
Another problem with conventional engines occurs because of incomplete mixture of fuel and air injected into the combustion chamber. If the fuel and air were mixed uniformly a leaner mixture could be used.